1. Field of the Invention
The present invention relates to a discharge lamp lighting apparatus for lighting a discharge lamp, such as a high-pressure mercury lamp, and a discharge lamp apparatus where the discharge lamp lighting apparatus and a discharge lamp are combined.
2. Description of the Related Art
The discharge lamp lighting apparatus converts the input power from the power supply into AC voltage and AC current for driving the discharge lamp, and supplies the AC voltage and AC current to the discharge lamp. The discharge lamp is driven by the AC voltage and AC current, and is lit. The discharge lamp tube voltage (AC voltage) of the discharge lamp fluctuates due to dispersion or time-based changes of the discharge lamp characteristics, but in the discharge lamp lighting apparatus, constant power control is performed for maintaining the discharge lamp tube power to be constant, regardless the fluctuation of the discharge lamp tube voltage.
However if the abovementioned constant power control is performed in an area where the discharge lamp tube voltage (AC voltage) is low, the discharge lamp tube current (AC current) suddenly increases as the discharge lamp tube voltage drops. The ratio of the factors which cause the loss in the discharge lamp lighting apparatus is high in a component depending on the discharge lamp tube current, and a sudden increase in the discharge lamp tube current causes an increase of loss in the discharge lamp lighting apparatus.
Also recently discharge lamp apparatuses are designed with less margin in cooling conditions to decrease the size and weight of the device, so an increase in the loss of the discharge lamp lighting apparatus easily causes a rise in the temperature of the discharge lamp lighting apparatus. If the temperature of the discharge lamp lighting apparatus rises, the overheat protection function of the discharge lamp apparatus is activated, which stops the supply of power to the discharge lamp and turns the discharge lamp OFF.
It is an object of the present invention to provide a discharge lamp lighting apparatus which can prevent an increase of loss in an area where the discharge lamp tube voltage is low, and a discharge lamp apparatus using the discharge lamp lighting apparatus.
It is another object of the present invention to provide a discharge lamp lighting apparatus which can prevent a rise in the temperature in an area where the discharge lamp tube voltage is low, and a discharge lamp apparatus using the discharge lamp lighting apparatus.
To achieve the abovementioned objects, a discharge lamp lighting apparatus according to the present invention comprises a power conversion circuit, a discharge lamp driving circuit, and a controller.
The power conversion circuit converts the input power to output DC power. The discharge lamp driving circuit converts the DC power supplied from the power conversion circuit to output AC voltage and AC current.
A signal corresponding to the AC voltage and a signal corresponding to the AC current are input to the controller. The controller provides constant power control for maintaining the AC power, which is provided by the AC voltage and AC current, to be constant to the power conversion circuit when the AC voltage is higher than a predetermined value.
The controller provides power reduction control for reducing the AC power to the power conversion circuit when the AC voltage is lower than the predetermined value.
In the abovementioned discharge lamp lighting apparatus according to the present invention, the input power is converted into DC power by the power conversion circuit. Moreover, this DC power is converted into AC voltage and AC current by the discharge lamp driving circuit, and is output. Therefore if the discharge lamp is connected to the output side of the discharge lamp driving circuit, the discharge lamp can be driven and lit by the AC voltage and AC current.
Also in the discharge lamp lighting apparatus according to the present invention, constant power control is performed for maintaining the AC power, which is provided by the AC voltage and the AC current, to be constant when the abovementioned AC voltage is higher than a predetermined value.
An important feature of the present invention is performing power reduction control to reduce the AC power when the AC voltage is lower than the abovementioned predetermined value. This power reduction control allows suppressing an increase of the AC current in an area where the AC voltage is low, which can prevent an increase of loss in the discharge lamp lighting apparatus.
Also if the increase of loss in the discharge lamp lighting apparatus in an area where the AC voltage is low is prevented, as mentioned above, a rise in the temperature of the discharge lamp lighting apparatus can be prevented as well, and activation of the overheat protection function and the discharge lamp turning OFF due to this activation can also be solved.
Also in the discharge lamp lighting apparatus according to the present invention, the abovementioned constant power control and the power reduction control are executed for the power conversion circuit which outputs the DC power, so these controls are simple.
It is preferable that the power reduction control has the characteristic to reduce the AC power according to the difference between the AC voltage and the abovementioned predetermined value. According to this power reduction control characteristic, an increase of the AC current in an area where the AC voltage is low can be suppressed effectively.
Generally in an area where the AC voltage (discharge lamp tube voltage) is low, the luminance of the discharge lamp does not drop very much, even if the AC power (discharge lamp tube power) is reduced slightly. Therefore the abovementioned power reduction control can be performed in a range where the luminance of the discharge lamp does not drop.
Another mode of the discharge lamp lighting apparatus according to the present invention may comprise a power conversion circuit, a high voltage generator, and a controller.
The power conversion circuit converts input power to output DC power. The high voltage generator receives the supply of the DC power from the power conversion circuit, and outputs DC voltage and DC current for driving a discharge lamp.
The controller, to which a signal corresponding to the DC voltage and a signal corresponding to the DC current are input, provides constant power control for maintaining the DC power, which is provided by the DC voltage and the DC current, to be constant to the power conversion circuit when the DC voltage is higher than a predetermined value, and provides power reduction control for reducing the DC power to the power conversion circuit when the DC voltage is lower than the predetermined value.
The discharge lamp lighting apparatus according to this mode as well exhibits a functional effect similar to the discharge lamp lighting apparatus described first.
Other objects, configurations and advantages of the present invention will be described in detail with reference to the accompanying drawings. The drawings, however, merely show examples.